Video color detectors have several different uses. The use of color detectors in color correction is described in the above-identified co-pending patent applications. The use of such detectors in chroma keying is described herein.
Chroma key devices and methods typically are used to insert images from one video source into the picture formed by signals from another source. Either of the sources can be any of a variety of different types, e.g., a television camera viewing a "live" scene; a telecine device producing video signals from motion picture film; a video tape player reproducing video signals recorded on magnetic tape or other media; etc.
A typical use of a chroma key device during a live television broadcast is illustrated in FIG. 8 of the accompanying drawings, which shows a video monitor or receiver 200 which displays a video picture 202. A video camera (not shown) views a weather forecaster 204 standing in front of a large vertical blue surface called a "matte" which is not visible in FIG. 8. Instead, wherever the blue matte appears behind the forecaster 204, it has been replaced by the image of a weather map 206 showing the U.S. and parts of Canada and Mexico. The forecaster sees the weather map image on a studio monitor (also not shown in FIG. 8) and points to areas of the matte to indicate areas on the map. The weather map can have superimposed images, such as temperature data, etc.
Video signals forming the weather map are generated by a video source usually referred to as a "background source", and the image of the weather forecaster is generated by the studio camera, which usually is called a "foreground source".
The insertion of a weather map in place of the blue matte is made by a chroma key device which detects the foreground signals from the video camera. When, during each horizontal scan, signals representing the blue color of the matte are detected, the chroma key device switches the broadcast signal from the foreground source to the background source. Then, when the non-blue color of the forecaster is detected, the signals broadcast are switched to the foreground source, and then back again to the background source when the matte is detected again. This process is repeated for each horizontal line in the picture. Thus, the background signal replaces the blue matte everywhere except where the forecaster is located.
A major problem with prior chroma key devices is that they often provide false key signals by responding erroneously to colors other than the matte color. For example, the device often produces key signals upon the detection of not only the blue matte signals, but also the blue eyes or clothing of the announcer. This is very undesirable.
Sometimes the chroma key device will not respond to the matte color. Then the lighting in the television studio or the matte must be changed so that the apparent matte color is just right for the chroma key device. This can be very time-consuming and expensive.
Variation in the matte color to which the chroma key device will respond is provided in some prior art devices by means of a variable hue or color control in which the color or hue to which the keyer will respond can be varied throughout the entire spectrum of visible light. FIG. 7 of the accompanying drawings is a schematic vector representation of the hue and saturation of a given color video signal, as it is shown on the familiar "vectorscope" used in video production equipment. The angle ".theta." of the vector represents the hue or color of the signals, and the length "S" of the vector represents the saturation or intensity of the signals. Although the hue or color control of such prior equipment theoretically makes it possible to select any hue as the matte color by, in effect, making it possible to rotate the vector in FIG. 7 through 360.degree., the keyer using this control actually responds to or "keys" on hues or colors far different from the desired matte color--at least when the colors are at anything less than high saturation levels. In other words, the device does not accurately discriminate among similar colors. This tends to cause false keying in response to other hues appearing elsewhere than in the matte area of the television picture.
The use of highly saturated or intense matte colors tends to somewhat alleviate the above problem, but places severe restraints on the colors usable in the studio, on lighting and other artistic variables, and on the variety of usable matte colors.
The use of prior chroma key devices also places undesirable restraint on the location of the weather reporter or announcer in relation to the matte. When using such prior devices, the announcer should be located at a relatively great distance from the matte so that blue matte color will not be reflected onto the announcer and cause false keying.
The use of bright or saturated blue as a matte color often causes "flare". Furthermore, the announcers usually must be smoothly coifed because the straggling hairs of a bouffant or other hair style with ragged edges sometimes will cause erroneous keying of the chroma key device.
Accordingly, it is an object of the invention to provide a video color detection device and method which are more accurate and flexible in use than in the past. It is a further object to provide a chroma key device and method which alleviate or solve the foregoing problems; a chroma key device and method in which the color to which the device responds can be controlled so as to prevent false switching or insufficient response to a desired color, and in which the color detection is more accurate and flexible than in the past.
It is another object of the invention to make it possible and practical to provide a plurality of different "keys" or insertions in a single television picture by switching among three or more video signal sources, one providing foreground and the others providing background signals for the multiple inserts.
It is a further object to provide a chroma key system and method capable of providing multiple keys in response to hues closely similar to one another.
An additional objective of the invention is to provide a color detection and chroma key device and method in which the degree of discrimination among hues can be adjusted so as to best adapt the operation of the device to the circumstances at hand.
Another object of the invention is to provide an improved hue detection device and method which is compatible with existing keying or switching devices having means for handling other chroma keying problems such as those caused by shadows, and the problem of providing either "soft" or "hard" edges for the key areas by providing either gradual or abrupt transition in switching between video sources, etc. Thus, it is an object to provide a chroma key device with excellent hue discrimination and other known chroma key features.
In accordance with the present invention, the foregoing objects ar satisfied by the provision of a color detection and chroma key devices and methods which are capable of discriminating among similar hues by limiting the band width of the hues and/or hue and saturation combinations which can be recognized by the device.
Preferably, this feature is provided in combination with a device which is capable of varying the center hue of the pass band in substantially infinitely small steps throughout the visible light spectrum. Thus, in essence, the vector representing the center of the pass band can be rotated through an angle of substantially 360.degree. in the vector plane so that substantially any hue can be used for the matte. However, if a narrow band width is selected, the device will not key falsely upon detecting any hue except those the same as or very close to the center hue of the pass band.
Preferably, the band width of the color detector of the device can be varied by the operator. Thus, the device can be made to discriminate against all but a narrow range of hues, as described above, or it can be made to respond to a relatively wide range of hues, if this is best for the particular scene involved.
The invention not only makes it possible to use almost any hue as a matte color, and to minimize false keying by selecting a narrow pass band, but it also eliminates or reduces the need for the use of very intense or saturated matte colors. In fact, an optional feature of the device makes it possible to limit the video signals to be detected to those having saturation values below a predetermined level. By this means, if two areas having the same hue but different saturation levels are present in the same picture, the keyer will discriminate between the two areas and make the required insertion in the low-saturation area. For example, if a motion picture film frame has both a light blue sky in one area and an intense or saturated blue automobile of the same hue in a different area of the frame, the device will make the insertion in the blue sky area only.
Conversely, if preferred, another optional feature can be used to limit the signals detected to those having saturation values above a pre-determined level so that, in the foregoing example, the insertion would be made only in the area occupied by the intense blue automobile.
By making it possible to use matte colors of relatively low saturation, several of the problems mentioned above are solved or alleviated. For example, colors of low saturation will reduce the reflection problems, with the result that the foreground object (e.g., the announcer) can be located closer to the matte. "Flare" and problems due to ragged coiffures also are alleviated, improving the flexibility of casting, hairstyles, etc.
The ability of the invention to discriminate between closely similar hues, or between high and low saturation signals of the same hues, greatly enhances the versatility of the chroma keyer.
In accordance with another feature of the invention, a plurality of detectors can be used to provide simultaneously a plurality of inserts into a given scene or frame of a video or motion picture. Since each detector can be adjusted to detect signals having a very limited range of hues and/or saturation values, the tendency towards false keying which would be created by the use of two or more prior chroma key devices in a single frame would be substantially alleviated.
In one embodiment of the invention, the matte color detector uses the same circuitry as that used in the color connector described in the aforementioned U.S. patent application Ser. No. 851,164, and can be used either as a component of a chroma key device, or in discriminating among different objects or areas of a frame of a motion picture or video tape so that only the color of the desired object or area will be corrected. Thus, the detector circuitry can be used for multiple purposes in film-to-tape, tape-to-tape, or tape-to-film transfers. Actually, when the chroma keyer is used with film or tape, an additional advantage is realized in that the matte area itself less frequently requires color correction to enable it to actuate the keyer. This saves color correction labor and time.
Further advantages are obtained by the use of a device and method in which controls are provided for defining a limited area or "window" in a video picture and limiting the colors sampled by a chroma key device to those within the limited area. This has the effect of limiting the appearance of the background picture to only a portion of the matte area. Preferably, the limited area is variable in both position and size, and can be programmed to move gradually from one frame to the next in order to follow movement of an object in tape-to-tape or film-to-tape transfers. This feature is particularly advantageous when combined with the improved color detector of the invention.
The chroma keyer of the invention also can be incorporated advantageously into production studio switchers, or video tape or film editing equipment, or it can be supplied as a stand-alone device cabled to cameras or other studio equipment.